Single Carb !!

Is there any good reason why, for a non-sportsbike, you couldn't have a
branched inlet manifold and just one large carburettor?
Anyone done this?
I was wondering about the XS.

 

Years ago, SU Carburettors offered just such a conversion for the old
Bonneville and Commando twins.

Worked very well - improved starting, virtually no less of performance
and better economy.

On a four-cylinder bike, making up the three-into-one manifold would be
tricky, and finding the space for a single (large) carb might also be
difficult, but there's no reason why not.

I suppose you'd lose some top end performance, probably gain mid-range,
and yes, improve economy. I've never seen it done, though.

 

How do cars do it? Or "did" would be better, seeing as all cars are FI
these days. I mean, cars managed happily for decades with (often) only
one carb.....

That's right, except that the carbs were twin-choke.

The Suzuki GSX550/750 did the same thing, with a pair of twin-choke
carbs, IIRC.

 

Triumph used single and dual carbs for different models, no reason why
others can't do the same. I know a guy that modified the manifold for his
Honda Goldwing and is running an Edelbrock 4 bbl carb. Bike starts and runs
good and sounds great with the straight pipes he made. It hesitates a bit
when he kicks in the 4 bbls though.

Les

 

Don't forget the GR650 Tempter, which also used a siamese carb.

 

Did it now? Cor, I rode one of those in, er, um, 1982 in Sweden and
never noticed. Horrible bike, mind.

 

Mea Culpa. I just looked at http://www.partsfish.com and it appears to
have used two conventional carbs.

I could have sworn Suzuki used one of those siamese carb thingys on a
twin, perhaps I misremember it from looking at the GSX550.

 

Both of my bikes and the pickup have one carb apiece. Of course, one of the
bikes only has one cylinder, while the other has two.

Having spent part of my youth listening to several little SU's each humming
their own tune, one carb is fine by me.

 

I'm expecting some carbs to arrive shortly for the XS but should
anything go amiss then I have been considering making up a three into
one manifold and using an SU carb and flat chrome filter.
I was just curious whether there was any damn good reason not to do
this.
Many Thanks
Should I have to give it a go then I'll get back with the result.

 

Ah, one of those technical types! All I ever had was a piece of vacuum hose
to listen to the whistle of each carb. Just like tuning a guitar. A twelve
string with a badly warped neck.

 

And you want to throw away 50 years of carburetor development and go backwards

Well em, NO, not really.

I suppose I just thought the SU was simple enough to start with for such
a project and as some of the Minis used twin SUs it would be possible to
get the delivery requirements fairly close.
I'd much prefer the proper carbs (and I may still end up fitting them)
but they are hard to find at the moment.
Anyway, very interesting reading.

 

I suppose I just thought the SU was simple enough to start with for such

SU carb is indeed very simple and reliable. The Minis which used pairs of
them used the small ones, 1-1/4 inch usually, and the perceived wisdom on
tunng the 'A'series engine is that twin carbs is a needless complication and
to use a single bigger one instead. On a 1275cc Mini this would be a single
1-3/4 inch item and the hot 1000cc Mini engine which I built used a single
1-1/2 inch carb.

SteveM

 

Problem with SU carbs is getting a lean cruising mixture and a rich power

There are also various different springs available to use above the piston,
I believe that there are also different grades of the damper rod that screw
into the top of the "bell" and which goes into the oil-filled cylindrical
extension of the top of the piston. There are, of course, literally hundreds
of different needles available.

I have to admit to liking SU carbs having been brought up on various British
cars that used 'em. That said I'd much rather have fuel-injection.

SteveM